What coordinates are chosen by default in Texture nodes?
As Christopher Bennett says in the comments, when you have no texture coordinate attached, it is picking a default for you, that default uses UV coordinates for mapping.
When you use Generated coordinates something interesting is happening.  It's attempting to map the image on the Z axis to your object, and since that axis doesn't exist for your two dimensional image, it's repeating the pixels at the edge of the texture along the Z axis of the object.